That's strange, the slideshow should always fetch the biggest version of the image around. I'll see if I can find out why the image quality would be worsened.

@QualityFramework

By default, the width of the slideshow is set to be the same width as the page it's on. You can set the width of a slideshow to an exact width by going into the admin, click on 'Slideshows' and then select the slideshow of which you want to alter the settings. There you'll find a slideshow's current settings, where you can define the 'Width of the slideshow' as many pixels as you want it to be in width.